Photovoltaic devices provide a non-polluting, silent and reliable source of electrical power. Originally, photovoltaic devices were fabricated from crystalline material and, as a consequence, were expensive and restricted in size. Techniques have now been developed for the preparation of large area thin film semiconductor materials which may be advantageously fabricated into low cost, large area photovoltaic devices.
It is frequently desirable to subdivide large area devices into a plurality of small area devices disposed upon a common substrate. Such arrays are generally referred to as monolithic modules. The structure of these modules makes them more tolerant of defects in the photovoltaic material and allows for the selection of desired voltage and/or current outputs. In some instances the small area devices of a monolithic array are interconnected in a series arrangement so as to provide for an increased voltage. In other instances it is desirable to provide an array of devices disposed in parallel. A parallel connected array provides a constant voltage which is independent of area. Furthermore, the presence of a large number of small area devices in the array allows for localization of defective regions and such defects may be readily removed without effect upon the remainder of the device.
In any type of photovoltaic device it is desirable to minimize areas which are not photovoltaically active. Such dead areas include portions of the device covered by grid lines, bus bars, electrical interconnects, or other such current-carrying members. In the fabrication of monolithic arrays it is necessary to scribe away portions of the photovoltaically active material to provide the plurality of isolated subcells and such scribed areas are also photovoltaically inactive.
Laser scribing techniques are often employed in the fabrication of monolithic photovoltaic arrays since a laser can scribe precise, fine lines in the photovoltaic device for a fairly low cost, thereby minimizing the expense of device fabrication and maximizing active area. Problems encountered in connection with prior art laser scribing techniques are generally attributable to the high levels of localized heating produced by the laser beam. Such heating can cause metal electrodes to melt and short circuit the device. Also, the laser can induce unwanted crystallization of amorphous semiconductor materials, thereby increasing their electrical conductivity and creating shunted current paths which degrade device efficiency.
It will be appreciated that it is desirable to eliminate, or minimize photovoltaically dead areas such as gridlines, bus bars, and other such current-carrying structures as well as scribe lines so as to increase active area and hence cell efficiency. It is also desirable to eliminate shunting, short circuiting, or other such damage occasioned by laser scribing of the layers of a photovoltaic device.
The present invention provides an improved configuration of monolithic array in which active area is minimized and the layers are disposed so that molten metal flow, semiconductor crystallization, and other such artifacts of the laser scribing process are significantly decreased. The present invention provides for the manufacture of a large area, monolithic array of interconnected small area photovoltaic devices. The array is economical to fabricate and manifests a high efficiency. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.